Skip Navigation to ContentSeattle Right-of-Way Improvement Manual SDOT Home
HomeSearch TipsRevisionsJoin Email ListGlossaryLinksContact UsFAQsdisclaimer
Chapter 4
  « Back to Chapter 3  |  Go to Chapter 5 »
Design Criteria
4.17 Drainage and Sewers

4.17.1 Links to Standard Plans and Specifications and Other Resources
4.17.2 Stormwater Code Compliance
4.17.3 Stormwater Collection  

  4.17.4 Drainage and Sewer Conveyance
4.17.5 Side Sewers
4.17.6 Additional Information

Street design includes provision for the collection, treatment and discharge of storm water. Drainage system components such as pipe, catch basins, and inlets and green stormwater infrastructure are considered integral street improvement elements as are curbs, sidewalks, street trees and pavement. All of the Street Drainage, Storm Drain and Sewer requirements in this section are to be considered requirements.

When property development includes the installation of new or replaced paving or other impervious surfaces, there may be a need to improve existing drainage systems in the street to accommodate the additional stormwater flows that will be incurred. This may trigger a requirement to make grading improvements, or to extend or upgrade existing storm drains, inlets, catch basins and sanitary sewer pipes, and there may be requirements for flow control and/or treatment facilities triggered by the Seattle Stormwater Code: Chapter 22.800 Stormwater Code. Factors such as the amount of impervious surface involved, the project location, site characteristics (to cover topography, soils, etc.) the availability and capacity of existing infrastructure, among others, play a role in determining these requirements.

All required drainage related improvements shall be designed and constructed in accordance with the City of Seattle Stormwater Code, Stormwater Manual and Standard Plans and Specifications which establish other requirements for pipelines, maintenance holes, connections, flow control and water quality facilities and other system improvements. In addition, Seattle Public Utilities' (SPU) CAM 1180 provides further guidance on the siting and design of drainage structures and conveyance facilities.

SPU is responsible for the plan review of new street drainage and stormwater treatment and flow control facilities, as well as for the review of project impacts to existing SPU infrastructure. This review typically happens through the Street Improvement Plan process for private development or during the project definition phase of an SDOT capital project.

The following design criteria apply for storm drains and street drainage including drainage appurtenances, bioretention cells, rain gardens and permeable pavements, sanitary and combined sewers,  and side sewers and service drains.

4.17.1 Links to Standard Plans and Specifications and other Resources

Standard Plans 200 Series; Sewer-Drainage Appurtances (2014)
Standard Specifications Division 5: Surface Treatments and Pavements (2014)
Standard Specifications Division 7: Storm Drain, Sanitary and Combined Sewers, Water Mains, and Related Structures (2014)
Standard Specifications Division 8: Miscellaneous Construction (2014)
Standard Specifications Divisions 9: Materials (2014)
Stormwater Code SMC Ch. 22.800
Stormwater Manual DPD DR 21-2015/SPU DR DWW-200 Volume III: Project Stormwater Control
Voluntary Roadside Rain Gardens and Bioretention SDOT CAM 2308 (2015) (Coming soon)
Design Guidelines for Public Storm Drain Facilities SPU CAM 1180 (2012)
Side Sewer Code – SMC Ch. 21.16
Requirements for Design and Construction of Side Sewers (Drainage & Wastewater Discharges) DPD DR 4-2011/SPU DR 2011-004
SPU Core Tap Procedures
Build-Over and/or Re-Route Review and Approval Process DPD TIP 507 (2015)

4.17.2 Stormwater Code and Manual Compliance

Project Type.  The Stormwater Code differentiates between projects on private property and projects in the right-of-way.  See DPD's Stormwater Code Reference Page.
   
It is sometimes challenging to construct surface stormwater elements while meeting the multiple other needs within the City's rights-of-way, including addressing public safety and public mobility needs.  The information provided in this section is intended to provide applicants with design guidance that has undergone review from various City departments involved in Street Improvement Permitting.  All sites are different so the project's engineer may need to modify the details provided to address local conditions. 

 
4.17.3 Bioretention Infiltrating and Non-Infiltrating Facilities and Rain Gardens

Bioretention cell designs may vary based on design goals and site conditions.  Generally all the cells include: surface grading, and soil and plant complexes to manage stormwater.  Factors influencing the design include: type of native soil, longitudinal and cross slopes, site context, presence or absence of curbs, locations of mature trees and space availability.  

The term bioretention is used to describe various designs using soil and plant complexes to manage stormwater. The healthy soil biology, soil structure and vegetation associated with bioretention facilities promote infiltration, storage, and slow release of stormwater flows to more closely mimic predeveloped conditions.
Rain gardens are shallow, landscaped depressions with compost amended soil or imported bioretention soil and plants adapted to the local climate and soil moisture conditions. Stormwater is stored as surface ponding before it filters through the underlying amended soil. Stormwater that exceeds the surface storage capacity overflows to an adjacent drainage system. Treated water is infiltrated into the underlying soil.

In the right-of-way, raingardens are typically used to just manage sidewalk runoff while bioretention is used to manage roadway runoff and larger drainage areas.

Design information and figures showing design requirements for infiltrating bioretention are shown in Section 5.4.4 of Volume 3: Project Stormwater Control, Stormwater Manual, DPD DR 21-2015/SPU DR DWW-200. Standard Plans 200 Series: Sewer / Drainage Appurtenance (2014) (see on-line Amendments), Standard  and Specifications Division 7, Section 7-21: Bioretention (2014).  

Design information and figures showing design requirements for rain gardens are included in Section 5.4.5 of Volume 3:  Project Stormwater Control, Stormwater Manual, DPD DR 21-2015/SPU DR DWW-200.

Voluntary rain gardens and bioretention proposed in the right-of-way are addressed in SDOT CAM 2308 Voluntary Roadside Rain Gardens and Bioretention. (Coming soon).

4.17.4 Permeable Pavements

Permeable pavement is a paving system which allows rainfall to percolate into an underlying soil or aggregate storage reservoir, where stormwater is stored and infiltrated to underlying subgrade, or removed by an overflow drainage system. Permeable pavement systems can be designed to provide differing degrees of flow control. Two categories of permeable pavement systems are included in the stormwater manual: permeable pavement facilities and permeable pavement surfaces.

Design information is provided in the Section 5.4.6 and 5.6.2 of Volume 3:  Project Stormwater Control, Stormwater Manual, DPD DR 21-2015/SPU DR DWW-200 , Standard Plans 400 Series: Street Paving & Appurtenance (2014) (see on-line Amendments), Standard  and Specifications Division 5: Surface Treatments and Pavements(2014).  


4.17.5 Setback/Clearance Requirements from Bioretention Cells and Raingardens

The following setbacks and clearances should be used for bioretention cells and rain gardens and associated appurtenances in the public right-of-way.

Category/Utility

Description of Setback/Clearance Requirement from bioretention cells and rain gardens

Overhead

Power poles

No power pole within bioretention cell or rain garden
5-ft (radius) level area around power poles.
No plantings except ground cover/sod within level area.

Guy wires

Typically 5-ft minimum clearance from top of bioretention cell or rain garden. However, more may be required by SCL if it is for a pole with transmission, primary or heavier load. Guy wire pole/anchor shall not be located in bioretention cell or rain garden.

Underground

Franchise/Electrical vaults

Not allowed within bioretention cell or rain garden section.

Underground duct bank for primary & secondary service

Not allowed within bioretention cell or rain garden section, unless approved by Seattle City Light / utility purveyor.

Electrical vaults and Bioretention cell vertical wall

See Seattle City Light standards for clearance requirements for different vault types.

Franchise vaults and Bioretention cell vertical wall

See Franchise Utility purveyors' standards for clearance requirements. General guidance: provide 5-foot clearance for future maintenance/construction clearance for wall/vault.

Underground electrical service conduit to a single-family residence (not encased in concrete)

May be located within the bioretention cell, rain garden or permeable pavement footprint  but prefer outside due to future maintenance.  When located within the bioretention cell footprint the electrical service conduit to route under the bioretention soil mix and supporting infrastructure (i.e. underdrain). Maintain SCL standard clearance from other underground piping.

Gas mains and services – Plan

May be located within the bioretention cell, rain garden or permeable pavement footprint but prefer outside for ease of maintenance access. Maintain standard clearance from other utilities.

Gas mains and services and vertical wall for bioretention cells

General guidance is to provide 5-foot clearance for construction of wall and/or future repairs of service/main, unless noted otherwise by utility purveyor (PSE).

Gas mains and services – Section

Maintain minimum cover over gas service/main per PSE. When located within the bioretention cell, rain garden or permeable pavement footprint the service shall be relocated to route under the bioretention section and supporting infrastructure (i.e. underdrain). Maintain standard clearance from other utilities.


Underdrains

Underdrain pipe cleanouts and bends

Cleanout required at/near bends used for routing underdrain around existing power poles and hydrants.

Underdrain pipe for bioretention cells & new street trees

If section of underdrain pipe is slotted then provide minimum five-foot horizontal separation between new street tree and slotted underdrain pipe.
If section of underdrain pipe is solid wall then provide minimum three-foot horizontal separation between new street tree and underdrain pipe.

Water Service & Meters

Water service and meter

Service may be located within the bioretention cell or rain garden footprint but prefer outside due to future maintenance.
Minimize locating bioretention cell or rain garden over water service piping that would require full replacement of the water service pipe (e.g. existing galvanized iron or plastic pipe) from the meter to the water main.
Meter shall be relocated to the back of walk in a landscape area w/n right-of way (preferred) or into the sidewalk.

Water service and meter crossing through bioretention cell section

Provide minimum depth of cover per SPU water.
If there is an underdrain, service may be above underdrain pipe as long as minimum depth of cover is provided.
Maintain standard clearance from other utilities per City Standard Plans and Specifications.
Meter/vault shall be located outside bioretention cell.

Water Mains

Water mains

Pipe cover: per Standard Plan 030.

Horizontal Clearance:

  1. From outside diameter of watermain: Minimum horizontal separation from bioretention cell and rain gardens are as follows:

    2"- 8" dia. Watermains -3ft- 0in
    10"– 12" dia. watermains 3ft – 6in
    16" dia.and larger watermains-Clearance requirements will be considered on a case by case basis with the Water LOB.

If soils must be disturbed within the horizontal clearances, a support plan for the watermain will be required and the disturbed soils shall require 95% compaction upon completion of construction.

  1. From centerline of watermain: Minimum horizontal separation from bioretention cell of 5-feet (measured at the horizontal point closest to the watermain).  If this setback results in loss of critical bioretention cell exceptions to this may be granted by SPU Water.

Zone of influence: Boiretention cells and/or rain garden sections shall not cross in to the zone of influence of the watermain.  Zone of influence of the watermain is a plane from the spring line of the pipe extending down and away from the main at a 1 to 1 slope.

Settlement Monitoring: Place bioretention cells and rain gardens such that settlement monitoring would not be required during construction. See Section 3.3 for reviewing requirements with SPU water Line of Business representatives. 

Water main/services and  vertical walls for bioretention cells

Review separation for horizontal clearance requirements with SPU water . General guidance to provide 5-foot clearance for maintenance/construction of vertical wall.

Water main cast iron with lead joints

Review horizontal clearance requirements with SPU water. Further clearance (x>5-foot) may be required for construction of vertical wall and protection of main.

Hydrants

Hydrants

Provide minimum 3 feet of cover over pipe to hydrant.
Provide 4-ft level area (4-ft radius from center of hydrant).
Limit plantings within 4 feet radius of hydrant except ground cover/sod around shear block. 
Not allowed within footprint of bioretention cell.
Minimize locating bioretention cells at locations that would require relocating a fire hydrant since this will increase project costs.  Relocating hydrants typically requires full replacement of hydrant lateral piping to the water main.

Signage

Signage

Locate signage for sight lines and in accordance with SDOT requirements.

Side Sewers/Service Drains

Side sewers/Service drains

May be located within the bioretention cell or rain garden or permeable pavement footprint  but shall maintain clearances from underdrain pipes if included in the design. Maintain standard cover and clearances.
Avoid locating infiltrating bioretention cells, rain gardens, and permeable pavements  over existing side sewers unless the side sewer is replaced since the stormwater could flow into the pipe at a crack or joint in the side sewer.
For non-infiltrating bioretention cells over existing side sewers, the bioretention cell section (including underdrain if used) shall be above side sewer.

Trees

New street trees

New street trees shall not be located within a lined bioretention cell. If the bioretention cell is lined, provide the following horizontal clearances between the root ball of the new tree and the outside of the liner:

  • Small trees: 3-foot horizontal clearance
  • All other trees: 5-foot horizontal clearance unless otherwise approved

In addition to the standard non-proprietary permeable pavements such as porous asphalt and pervious concrete, certain permeable pavers may be allowed to be installed in the right-of-way at the discretion of the City. See table of Allowable Permeable Pavement Wearing Course Materials for Stormwater Credit.

4.17.6 Stormwater Collection

Drainage Infrastructure: Shall be provided for improved streets and alleys and shall include catch basins and inlets that discharge to a public combined sewer, public storm drain, or other discharge point approved by Seattle Public Utilities (SPU).

Surface drainage: Standards for cross slopes and longitudinal slopes are important for vehicular and pedestrian safety as well as surface water conveyance. Cross slope shall ensure surface drainage gets to the gutter or flow line and flows down to drainage pickups. Streets shall generally have a centerline crown elevation, with some exceptions, such as super-elevated streets. For alternative street grading concepts, designers have looked to the WSDOT Hydraulics manual for guidance on evaluating flow spread along the gutter line. Refer to Standard Plans 200 Series, Standard Specifications Division 7 and SPU CAM 1180 for more information.

 

4.17.7 Drainage and Sewer Conveyance

Public storm drains: A new public storm drain may be required when

  • there is no available public storm drain and there is no acceptable discharge point; and
  • to achieve adequate capacity;

New public storm drains shall be sized to handle all upstream tributary area from the drainage basin in which they are located. They shall be designed to be continuous with existing and future storm drain pipes that are or will be part of the storm drainage system. The engineer shall provide the hydraulic calculations used in the pipe design for review by SPU. Additional information is included in SPU CAM 1180 Design Guidelines for Public Storm Drain Facilities (2012).

 

4.17.8 Sanitary and Combined Sewers

In general, requirements for sanitary and combined sewer mains are as described above for storm drain pipes with the following differences:

  • All sewer pipe must be designed and constructed to give mean velocities, when flowing full, of not less than 3 fps. 
  • Minimum pipe diameter is 8 inches
  • Manholes are required every 350 feet.
Standard pipe depth is 12 feet.
4.17.9 Side Sewers and Service Drains

Side sewers and service drains:  The pipeline between the building and the sewer or storm drain main is a side sewer or service drain, respectively.  All side sewer and service drain work within the right-of-way requires a Street Use Utility Permit issued by DPD on behalf of SDOT. Additional information is available in Seattle Municipal Code: Chapter 21.16 Side Sewer Code and DR 4-2011 Requirements for Design & Construction of Side Sewers (Drainage & Wastewater Discharges)

Minimum grade, pipe size and materials, connection details, installation and testing requirements:  Side sewers and service drains shall be designed and installed in accordance with the City of Seattle Standard Plans and Specifications. New core taps shall be per SPU's Core Tap Procedure.

Ownership:  Side sewers and service drains are owned and maintained by the property owner. The minimum grade is 2%.  Pipe material and size shall be according to the Seattle Municipal Code 21.16 – Side Sewers.  

Use of existing side sewers:  It is possible to use existing side sewers in lieu of a new connection in some cases See the Side Sewer code for more details.

4.17.10 Additional Information

Build-Overs: For build-overs for SPU owned sewer and drainage appurtenances, the applicant shall follow the guidance of DPD's Build-Over and/or Re-Route Review and Approval Process: Tip 507 (2015).

Ditch Modifications: The City does not permit the filling of a ditch if that ditch functions as part of the City's informal drainage system in the street right of way and is located within a creek watershed.  Creek watersheds are identified on the City GIS system, and on the Seattle Creek Watersheds map.  The SDOT Director may approve a requested exception per ROWIM Section 2.11 or the Street Use Code SMC 15.04.112 (Decisions – review or reconsideration)

Deviation from Drainage and Wastewater Standards: Requirements for sewer and storm drain extensions should be identified in the preliminary application process and appeal issues should be resolved prior to development of street improvement plans. Deviations from drainage and wastewater standards should be identified and resolved during design guidance to avoid potential delays during formal review and/or construction.
continue to section 4.18 »   
Latest Online Manual
Detailed Table of Contents
Chapter 4
Design Criteria
4.1 Introduction
4.2 Street Classifications and Street Types
4.3 Design Criteria General Notes
4.4 Grading
4.5 Design Cross Section
4.6 Roadway Width
4.7

Roadway Pavement

4.8 Intersections
4.9 Driveways
4.10 Curbs
4.11 Sidewalks
4.12 Crosswalks
4.13 Bicycle Facilities
4.14 Street Trees and Landscape Architectural Standards
4.15 Introduction to Utilities Design Criteria
4.16 Street Lighting
4.17 Street Drainage, Storm Drains and Sewers
4.18 Water Mains
4.19 Fire Protection
4.20 Seattle City Light
4.21 Clearances
4.22 Structures in the Right-of-Way
4.23 Culdesacs and Turnarounds
4.24 Traffic Operations
4.25 Transit Zones
4.26 Street Furniture, Public Art and Unique Objects in the Public Right-of-Way
4.27 Access Easements
4.28 Contact Information
   
Report ROWIM content errors and/or website problems Report ROWIM content errors and/or website problems
Download Adobe Acrobat Reader Download Adobe Acrobat Reader

 

End of Content
Home | Search Tips | Revisions | Join Email List | Glossary | Links | Contact Us | FAQs
SDOT Home | Seattle.Gov